This invention relates generally to sealed micromachined sensors, actuators, and structures such as capacitive pressure sensors and, in particular, to electrical lead transfer technologies and methods of realizing feedthrough structures from inside a sealed cavity to an area outside that can be conveniently accessed.
One of the most difficult problems in the design, fabrication, and commercialization of micromachined sensors and actuators resides in the realization of electrical feedthroughs that can be conveniently accessed outside of a sealed cavity of micromachined device(s). The sealed cavity can play a few major roles, including but not limited to housing the micromachined device, a reference cavity, and a sensing cavity. Although several electrical lead transfer techniques have been developed for capacitive absolute pressure sensors, the designs present technical drawbacks that limit their applicability.
One approach uses a pn-junction feedthrough. In this technique, n-type silicon is used as the silicon substrate and it is then diffused to create a p+ silicon diaphragm (the upper electrode). The bottom electrode of the pressure measurement capacitor is made by two separate metal patterns on the glass substrate wafer. The two metal parts are connected during the anodic (electrostatic) bonding process via the diffused p+ feedthroughs which are fabricated on the N-type silicon. The technique, however, suffers from major problems associated with P-N junctions. For example, it exhibits a high temperature dependence and drift as well as junction noise and reverse leakage currents. As a result, this technique fails to provide high quality, reliable feedthroughs at a reasonable cost.
An improved approach utilizes polysilicon electrical feedthroughs in conjunction with a deposited dielectric layer, such as silicon oxide or silicon nitride. Anodic bonding is used between polysilicon (and silicon dioxide) and a glass substrate, as opposed to single-crystal silicon and a glass substrate. Although the feasibility of this approach has been proven, the hermetic quality of the anodically bonded polysilicon to the glass warrants further investigation. Also, the fabrication process is quite complicated and surface non-uniformity may adversely affect yield.
U.S. Pat. No. 4,386,453 to Giachino et al. discloses a capacitive pressure sensor fabricated from a silicon wafer and a dielectric (glass) substrate using anodic bonding. Holes formed in the glass to provide electrical contact are subsequently sealed with solder. The major problems of this approach include low quality of sealing and outgassing of the solder refill, resulting in a reduced-vacuum pressure within the cavity.
U.S. Pat. No. 4,773,972 to Mikkor describes a method of fabricating a capacitive pressure sensor by anodically bonding two silicon wafers, each containing a capacitive plate. The process is quite complicated, including multiple etch steps and temperature-controlled conductor migration, resulting in challenges at the manufacturing level and above-vacuum pressure inside the cavity.
U.S. Pat. No. 5,264,075 avoids the need to drill holes or form complex conductive paths through a dielectric material, and also eliminates the use of solder plugs. The sensor is comprised of a silicon substrate bonded to a dielectric (glass) substrate. The silicon substrate includes a diaphragm formed by front- and back-side etching. This technology as described in this patent appears to be susceptible to certain yield-reducing steps. In particular, because sputtered material is used in the process, hermetic quality, step coverage, and outgassing inside the cavity are questionable.
U.S. Pat. No. 5,528,452 discloses a sensor constructed of two major components, a silicon diaphragm and a glass substrate. In comparison with the other techniques described above, the approach taught by this patent affords a much simpler fabrication process. However, bonding is used in conjunction with a deposited layer of reflow glass, rather than directly bonding to a glass wafer. Furthermore, metal lines penetrate through the silicon rim, creating problems relating to step coverage.
Monolithic Sensors Inc. has developed a capacitive sensor system made entirely of silicon, which minimizes problems associated with the thermal coefficient of expansion. The sensor incorporates a top plate having a mechanical pressure stop (silicon); a silicon diaphragm; and a back plate (also silicon) having CMOS circuitry. The three components are fabricated separately, and bonded together through eutectic soldering. Although the resulting system features a wide band of linearity, it is intended for differential pressure applications where absolute sealing is not required. Another non-sealed configuration is disclosed in U.S. Pat. No. 5,381,299, which provides a through-hole that allows a fluid whose pressure is being measured to contact the diaphragm.
Other techniques possess different drawbacks in terms of design, development, fabrication, manufacturing, and/or overall cost. In one case, a pressure sensor along with a glass frit of proper composition is placed into a vacuum furnace and heated to a defined temperature-time curve to melt the frit, resulting in a process which is very difficult to control. Another approach seals the reference cavity with a wafer-level process by sputtering Pyrex glass film over a channel opening. Due to these drawbacks in the prior art, the need remains for a low cost, reliable, hermetic sealing, and wafer-level feedthrough fabrication technique applicable to the packaging and manufacturing of micromachined devices.
This invention is directed toward structures and methods for the fabrication of electrical lead transfer feedthroughs with respect to a sealed cavity. The cavity may be evacuated or filled with specific gases at specific pressures. As such, the invention finds application in the packaging (vacuum or controlled environment) and production of a variety of transducers including but not limited to absolute pressure sensors, differential pressure sensors, flow sensors, mass flow controllers, fluidic products (e.g., drug delivery systems, valves, and chemical analysis systems), optical devices (e.g., infrared detectors, CCD camera, and flat-panel displays) and resonating devices, such as gyroscopes, accelerometers, yaw sensors, mechanical and electrical filters, oscillators, etc. Multiple lead transfer feedthroughs may also produced using this invention.
In a structure of the type wherein the bottom surface of a top substrate (which is usually but not necessarily substantially electrically conductive) is attached to the top surface of a bottom substrate (which is usually but not necessarily substantially electrically insulating) to provide the sealed cavity, an apparatus aspect of the invention provides means for interconnecting an electrically conductive element outside the cavity to an electrically conductive element within the cavity using a feedthrough section of the top substrate which makes electrical contact to both of the electrically conductive elements and an electrically insulating barrier formed in the top substrate to electrically isolate the feedthrough section from the rest of the top substrate. In a preferred embodiment, only a portion of the bottom surface of the feedthrough section of the top substrate overlaps with portions of the electrically conductive elements, enabling the entire rim to be bonded directly to the top surface of the substrate using techniques such as anodic bonding to ensure vacuum integrity where desirable. These techniques can be applied by two different categories: electrical access from top side of the top substrate, and access from the back side of the bottom insulating substrate via through holes in the bottom substrate (e.g., glass).
In some applications such as capacitive pressure sensing, the top substrate in the vicinity of the feedthrough section may include an outer wall, in which case the electrically insulating barrier is preferably U-shaped, with the ends of the U terminating at the outer wall. In another important embodiment, the feedthrough section is in the form of an island of conductive material surrounded by the electrically insulating barrier, preferably assuming an O-shaped island available for subsequent interconnection. This island is completely surrounded by both the electrically insulating barrier and the rest of the top substrate.
A method of interconnecting an electrically conductive element outside a sealed cavity to an electrically conductive element within the cavity according to the invention would preferably include the steps of:
providing a bottom substrate (which is usually but not necessary substantially electrically insulating) having top and bottom surfaces;
providing a top substrate (which is usually but not necessary substantially electrically conductive) having top and bottom surfaces where the bottom surface is adapted for attachment to the top surface of the bottom substrate to provide the sealed cavity;
electrically isolating a portion of the top substrate from the rest of the top substrate, the isolated portion having a top surface outside the cavity and a bottom surface facing the top surface of the bottom substrate;
connecting the electrically conductive element outside the cavity to either (1) the top surface of the electrically isolated portion of the top substrate, or (2) the bottom surface of the electrically isolated portion of the top substrate via through holes in the bottom substrate; and
connecting the electrically conductive element inside the cavity to the bottom surface of the electrically isolated portion of the top substrate, such that the electrically isolating portion of the top substrate acts as a conductive link between the elements.
In yet another aspect of this invention, an innovative technique is introduced for refilling etched trenches which is high yield, low cost, and high quality.
The scope of the applicability of the present invention will become more apparent from the following detailed description. It should be mentioned, however, that the detailed description and specific examples, while indicating the preferred embodiments of this invention, are given by way of illustration only. As a result, various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in this field.